1. Field of the Invention
The present invention relates to the recognition and separation of carbon clusters. In one aspect, the invention is directed to a recognition selector useful, for example, as a stationary phase in the chromatographic separation of carbon clusters, such as C.sub.60 and C.sub.70 fullerenes. In another aspect, the invention is directed to the use of elevated temperatures to obtain improved separation of carbon clusters, such as C.sub.60 and C.sub.70 fullerenes.
2. Description of the Prior Art
Solid, or elemental, carbon has for some time been thought to exist in only two forms: diamond, wherein the structure is face-centered cubic with each carbon atom bonded to four other carbon atoms in the form of a tetrahedron: and graphite, which has layers of carbon atoms arranged in the form of hexagons lying in planes, the carbon atoms of various layers being aligned with each other.
Recently, a third form of solid carbon has been discovered. Unlike diamond and graphite, however, this new form of carbon has the structure of a closed shell. This family of closed carbon shells, also referred to herein as carbon clusters, include for example that group of closed carbon shells denominated in the art as the fullerenes, such as those highly stable molecules known as Buckminsterfullerene, and the related molecule known as fullerene-70; other members of the fullerene family include, e.g., C.sub.78 and C.sub.240 .
Buckminsterfullerene, also known as C.sub.60, is a 60 carbon atom molecule having the geometry of a truncated icosahedron; that is, a polygon with 60 vertices whereat the carbon atoms are placed, and 32 faces, 12 of which are pentagons and 20 of which are hexagons. C.sub.60 thus has the geometry of a soccerball.
Fullerene-70, also known as C.sub.70, is similar to C.sub.60, only it has 10 additional carbon atoms which are believed to be inserted as a band of hexagons around the middle of the truncated icosahedron.
Although only recently discovered, various routes for synthesizing fullerenes are already available. An aspect of fullerene production that is of especial importance is the separation of fullerenes, as a whole, from a resultant fullerene-containing product admixture, as well as the separation of the various species of fullerenes from each other.
Presently known separation techniques include the method described by Taylor, et al. in J. Chem. Soc., Chem. Commun., pp. 1423-1425 (1990). This method employs solvent extraction to remove C.sub.60 and C.sub.70 from carbon product deposits, followed by chromatographic separation using alumina and hexane to separate C.sub.60 from C.sub.70.
Another approach, which is reported by Hawkins, et al. in J. Org. Chem., 55, pp. 6250-6252 (1990) entails the use of flash chromatography by dry loading onto silica gel, and elution with hexanes to achieve a 40% recovery of material that is almost exclusively C.sub.60 and C.sub.70. C.sub.60 and C.sub.70 are then separated chromatographically using a commercially available N-3,5-(dinitrobenzoyl)-phenylglycine derived stationary phase eluted with hexane. Cox, et al. have also described the chromatographic purification of C.sub.60 and C.sub.70 using a K-acidic dinitroaniline stationary phase in J. Amer. Chem. Soc., 113, pp. 2940 (1991).
Notwithstanding these developments, the methods for separating and purifying fullerenes, as known heretofore, have been only marginally successful insofar as practical analytic and, more importantly, preparative scale applications are concerned. For example, the chromatographic separations that are described by Taylor, et al. and Hawkins, et al. are impractical for preparative scale preparation of fullerenes because of the extreme insolubility of the fullerene analytes in the mobile phase. This condition limits, for example, the sample sizes which can be separated per run.
Thus there continues to be a pressing need for a method of separating fullerenes in a manner that is qualitatively and quantitatively superior to known methods, and that will be of practical utility in both the analytic and preparative scale separation of carbon clusters, such as C.sub.60 and C.sub.70 fullerenes.